Characterization of quartz in the Wufeng Formation in northwest Hunan Province, south China and its implications for reservoir quality

Journal of Petroleum Science and Engineering 179 (2019) 979–996

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Characterization of quartz in the Wufeng Formation in northwest Hunan Province, south China and its implications for reservoir quality

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Zhaodong Xia,b,c, Shuheng Tanga,b,c,∗, Songhang Zhanga,b,c, Yongxiang Yia,b,c, Feng Danga,b,c, Yapei Yea,b,c a

School of Energy resource, China University of Geosciences, Beijing, China Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism, Ministry of Education, China c Key Laboratory of Strategy Evaluation for Shale Gas, Ministry of Land and Resources, China b

ARTICLE INFO

ABSTRACT

Keywords: Authigenic quartz Silica source Marine shale Reservoir Diagenesis Pore structure

Quartz is one of the most important and common minerals in marine shales. This is especially true for the Wufeng-Longmaxi and Niutitang Formations in south China. However, the type, origin and formation mechanism of the quartz in these marine shales are as yet unclear. Thirteen samples of Ordovician shale from the Wufeng Formation in northwest Hunan Province were collected for this study. The quartz within the Wufeng shale was characterized through a series of experiments employing scanning electron microscopy with cathodoluminescence, nitrogen physisorption, X-ray diffraction, and geochemical analysis. The results were then used to assess the implications of the quartz component for reservoir quality. Three quartz types were identified based on scanning electron microscopy and cathodoluminescence imaging. Type I quartz has a relatively large grain size and exhibits bright luminescence, indicative of a terrigenous origin, whereas Type II and Type III quartz is authigenic and has dull or no luminescence. Type II quartz has small grain sizes and may be mainly biogenic in origin. This type of quartz may have formed during early diagenesis, with the silica being sourced from the dissolution of siliceous organisms. Type III quartz generally appears as dark rims on Type I quartz grains; the silica may have been derived from clay mineral alteration, with quartz-formation occurring during middle or late diagenesis. The authigenic quartz (Types II and III quartz) likely formed as a cement and would be expected to generate brittle behavior. The shale with a high content of authigenic quartz has a high Young's modulus and low Poisson's ratio. The biogenic authigenic quartz (Type II quartz) was pore-filling and formed during shallow burial. It would thus have been able to resist further compaction and provide a rigid framework within which the primary pores in the shale could be preserved, controlling the distribution of organic matter and the development of organic pores. The biogenic authigenic quartz has positive relationships with porosity and pore volume and specific surface area. Shale that was high in biogenic quartz had high porosity and adsorption capacity. The characterization of quartz in marine shales has significance for shale gas exploration and development and provides a new angle for evaluating the fracability and storage capacity of shale gas reservoirs.

1. Introduction Shale gas has been under exploration and development in China for 12 years. It has gone through four developmental stages: initial geological research on shale gas, selection of ‘sweet spots’ and preparation for early exploration and development, pilot tests for industrial extraction of marine shale gas, and evaluation and exploration of nonmarine shale gas (Zou et al., 2015; Zhao et al., 2016). At present, marine shale in the Wufeng, Longmaxi and Niutitang Formations is under commercial development in south China. Shale gas development has seen most success in the Sichuan Basin, where three major shale gas ∗

fields have been established in the Fuling, Changning and Weiyuan areas (Xi et al., 2018c). The geological reserves are 5441.29 × 108 m3, and the output exceeded 78 × 108 m3 in 2016. Future development is expected to achieve 300 × 108 m3 to 600 × 108 m3 (Ma et al., 2018). As shale gas reservoirs have been characterized as self-contained source–reservoir systems (Chen et al., 2011), reservoir quality and the factors influencing it have been discussed extensively and in detail. The components of the shale, primarily organic matter (OM), quartz, and clay minerals, are considered to be one of the most important controlling factors affecting shale gas formation, accumulation, and exploitation (Jiang et al., 2017; Wang et al., 2016; Tan et al., 2014).

Corresponding author. School of Energy resource, China University of Geosciences, Beijing, China. E-mail address: [email protected] (S. Tang).

https://doi.org/10.1016/j.petrol.2019.04.051 Received 27 November 2018; Received in revised form 19 February 2019; Accepted 14 April 2019 Available online 23 April 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The distribution of sampling well SY5 in northwest Hunan from southern China (Modified from Xi et al., 2018b).

Milliken et al. (2016) proposed that much authigenic microquartz can be observed in carbonate-rich mudrocks in the Eagle Ford Formation and that much of it has a biogenic origin. This finding has great significance for mudrock classification. In addition, a better understanding of quartz in mudstones has many other practical applications, including in sequence stratigraphy, petroleum exploration and exploitation, hydrogeology, and archeometry (Milliken et al., 2016; Niu et al., 2018). Thus, many geological and mineralogical investigations have recently set out to study the specific properties of quartz in marine shales. There are many experimental methods that can describe quartz in marine shales (Blatt and Schultz, 1976; Gotze et al., 2001; Lehmann et al., 2009; Buckman et al., 2017). In general, discrimination of quartz origin depends on the use of scanning electron microscopy (SEM), cathodoluminescence (CL) or X-ray mapping in combination with other methods such as geochemical and oxygen isotope analysis (Schieber et al., 2000). Therefore, the nature of quartz in the Ordovician Wufeng Formation of south China and its implications were characterized through SEM/SEM-CL imaging, nitrogen adsorption, and geochemical analyses. The aims are to determine quartz types and origins in the Wufeng marine shale and discuss the impacts of quartz on the pore structure and brittleness of shale gas reservoirs. The data and information in this paper has great theoretical significance for understanding the role of quartz on shale gas reservoir quality and will offer practical guidance for marine shale gas exploration and development.

However, many studies have ignored the grain assemblages and diagenesis of marine shales and their impact on reservoir quality (Milliken and Day-Stirrat, 2013; Zhao et al., 2017, 2018; Li et al., 2018b; Liu et al., 2018). There has been a lack of research into how the primary detrital composition affects shale property evolution, how the formation and distribution of shale components are related to burial history, and whether shales with different grain assemblages have different porosities, degrees of permeability, organic matter (OM) pore characteristics, or mechanical properties. The grain assemblage and diagenesis are already recognized as having important impacts on sandstone reservoirs and their investigation is an important research direction, which may indicate their importance as a research direction for shales also. Quartz is one of the most important components of marine shales. It is also the most abundant and important silicate mineral in the Earth's crust and occurs in a variety of diagenetic forms (Potter et al., 2005; Passey et al., 2010; Milliken et al., 2012). Quartz in marine shale has diverse origins (extrabasinal detrital and intrabasinal authigenic) and size (sand, silt or clay size) and shape because silica sources and quartz formation mechanisms are extremely varied and complicated in marine environments (Zhao et al., 2017a). Milliken (1994) found rims of weakly luminescent quartz on some silt-sized quartz grains in Oligocene mudrocks. Dowey and Taylor (2017) found that there are some individual authigenic quartz grains, generally 2–3 μm, which developed in a fine-grained clay matrix. Niu et al. (2018) found some small authigenic quartz filling sponge spicules or algal cysts. Quartz with different formation mechanisms and origins has different properties, which can significantly affect the reservoir quality (Zeng et al., 2013; Xi et al., 2018b). However, relatively few studies have characterized the quartz in marine shales of the early Paleozoic in detail, and the implications of the nature of the quartz for reservoir quality remain unclear. In general, TOC content has positive relationships with biogenic quartz and negative correlation with detrital quartz as indicated by Bustin et al. (2009), and biogenic authigenic quartz can work in concert with OM, which is conducive to pore development. Zhao et al. (2017) pointed that most of authigenic quartz were biogenic in the WufengLongmaxi Formations in the Sichuan Basin. They suggested that OM pores are more developed where the shale contains more authigenic quartz. Harris et al. (2011), meanwhile, found that the Upper and Lower Members of the Woodford shale have different mechanical anisotropies and that this can be attributed to the quartz within them. The Upper Member contains more biogenic quartz, making the shales in the Upper Woodford significantly more brittle than those in the Lower.

2. Geological setting Hunan Province is at the periphery of Sichuan Basin and incorporates the Sangzhi-Shimen synclinorium of the western Hunan and Hubei fold belt (Fig. 1; Xi et al., 2018b). Organic-rich black shales of the Ordovician–Lower Silurian Wufeng-Longmaxi Formations in which shale gas has accumulated are widespread in northwest Hunan Province (Wan et al., 2017). The thickness of the Wufeng-Longmaxi Formations generally decreases towards the southeast in this area, controlled by a change from shallow shelf to deep shelf sedimentary facies (Wan et al., 2017). In vertical, the Lower Member of the Wufeng-Longmaxi Formations mainly developed siliceous shale and carbonaceous shale (Li et al., 2016), whereas the Upper Member of the Longmaxi Formation mainly consists of dark gray shales, silty shale and gray-white argillaceous siltstone (Fig. 2). Well SY5 was drilled at the Sangzhi County in the Hunan Province in 2016 to evaluate shale gas potential in the Wufeng-Longmaxi Formations in Hunan Province. Well SY5 revealed the Wufeng Formation spanning 1083–1095 m and the Wufeng 980

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(Waychunas, 1988; Gotze et al., 2001; Frelinger et al., 2015). Sippel (1968) was the first to use CL images to discriminate quartz types and their paragenetic history in quartzose sandstones. The applicability of CL images for describing quartz was soon widely adopted, and many papers have been published on this topic (Corfu, 2003; Milliken et al., 2005, 2007). Quartz of different types and origins emit different CL intensities and colors because of differences in their basic crystal lattice or the presence of trace element substitution or crystal defects (Zinkernagel, 1978; Frelinger et al., 2015; Dowey and Taylor, 2017). Generally, detrital quartz has bright luminescence and variable colors and authigenic quartz has weak or no luminescence. Inspection of CL spectra in the wavelength range 320–850 nm under standardized conditions is often used to further characterize quartz (Gotze et al., 2001; Zhao et al., 2017a). Natural quartz has emission bands in this spectral region, and the relative intensities of the dominant peaks can be used to identify quartz type and origin. Authigenic quartz usually produces a broad luminescence peak with a major emission peak around 620 nm, whereas detrital quartz usually has two peaks. CL imaging for this study was conducted using scanning electron micro-beam instruments with cathodoluminescence (Mono CL detector). An EG&G digital triple-grating spectrograph with an LN2cooled CCD detector was used to obtain the CL spectra. It should be noted that quartz in marine shales is finer-grained than in sandstones, and even a high-resolution SEM-CL instrument barely covers the range required for observation of quartz in such fine-grained sedimentary rocks, which are, at most, a few microns in diameter. 1–5 μm quartz grains can be imaged clearly under ideal circumstances (Milliken and Day-Stirrat, 2013). Grey-scale CL imaging rather than color CL imaging was obtained for this study because a color CL total imaging scan takes about 10 min, whereas grey-scale CL imaging only needs 60–90 s and is sufficient for identifying the quartz types in this study (Schieber et al., 2000; Xi et al., 2019).

Fig. 2. Geological drilling histograms of Wufeng-Longmaxi Formations in northwest Hunan.

Formation mainly developed black carbonaceous shale (Fig. 2). 3. Samples and methods

4. Results

3.1. Samples

4.1. Mineral composition and organic geochemistry

Thirteen marine shale samples from the Wufeng Formation were collected from well SY5. Each sample was subjected to a series of experiments: the origin and types of quartz were determined by imaging using scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), and scanned cathodoluminescence (CL) and by analysis of CL spectra. Additionally, vitrinite reflectance analysis, XRD analysis, Rock-Eval analysis, X-Ray fluorescence spectrometer (XRF), nitrogen (N2) adsorption experiment, porosity determination, methane adsorption and rock mechanical analysis (Young's modulus and Poisson's ratio) were conducted to characterize the shale gas reservoir on all the thirteen samples. Most of the methods, experimental apparatus and processes used in this paper were the same as used in our previous studies, and thus detailed information on these experimental methods can be found in the references (Xi et al., 2018b, 2018d) and are not discussed further in this paper. Although all the thirteen samples were collected from a single well and their depth is variable, the Wufeng Formation only 12 m thick and thus the depth of the samples varied irregularly within a narrow range.

Quartz and clays are the dominant minerals in the Wufeng shales, comprising, on average, 55.61% and 18.63% of the rock, respectively. The next most common minerals are feldspars, averaging 12.42% of the rock (Table 1). The quartz content ranged from 42.3% to 69.8%, and the clays ranged from 10.6% to 26.5%; these proportions are similar to those in other Wufeng-Longmaxi shales in the Sichuan Basin (Chen et al., 2011) but a little lower than those of the Niutitang Foramtion (Xi et al., 2018b). Illite is the primary clay mineral type, averaging 15.23% of the rock, followed by illite/smectite (I/S), averaging 3.39% (Table 1). Representative X-ray patterns from the Wufeng shale are displayed in Fig. 3. All the thirteen samples had three types of quartz peak. The primary peak was at 0.334 nm in d-space (26.64°) and may relate to the quartz (101) diffraction peak, the secondary peak was at 0.426 nm in d-space (21.78°), potentially relating to the quartz (100) diffraction peak, and the third type comprises several weak peaks at the right side of Fig. 3, which may relate to the diffraction peaks of synthetic α-quartz. In addition, there is a small peak at 0.43 nm in d-space (20.98°) for sample SY-5 (Fig. 2); this may relate to the diffraction peak of opal or α-tridymite (which originates from opal dehydration) (Flörke, 1955; Wilson et al., 1974). This small peak was not observed in the other samples in this study and is rarely observed in other studies of thermo-mature shales (Kumar et al., 2018). The abundance of OM was between 1.58% and 4.25%, with an average of 2.71%; amorphous sapropel is the dominant OM maceral, accounting for more than 80% (Table 1). The TI values, which fall between 40 and 80, indicate that the kerogen in the Wufeng shales is primarily Type II (Cao, 1985). The vitrinite reflectance (Ro) value was between 2.13% and 2.84% with an average of 2.55%, and the average value of Tmax was 475 °C, which indicates that the shale is in the over

3.2. Cathodoluminescence and its application for quartz SEM-CL imaging can be used to identify quartz types and infer the genesis of quartz in mudstones (Schieber et al., 2000; Li et al., 2018b; Niu et al., 2018). It is of particular interest because the information regarding quartz in mudstones obtained by cathodoluminescence is not obtainable through other analytical methods (Milliken and Day-Stirrat, 2013). Cathodoluminescence is the light emission produced by accelerated electron excitation and originates from the interaction between the excited electrons and the shell electrons in the luminescent materials. This method is widely used to inspect geological materials 981

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Table 1 The results of Rock-Eval analysis, XRD analysis, and maceral composition analysis. Sample ID

SY-1 SY-2 SY-3 SY-4 SY-5 SY-6 SY-7 SY-8 SY-9 SY-10 SY-11 SY-12 SY-13

Depth (m)

1083.4 1084 1084.8 1085.6 1086.4 1087.2 1088.2 1088.9 1089.6 1090.3 1091 1091.6 1092

TOC (%)

2.55 3.03 4.05 2.35 2.39 1.88 2.31 1.95 2.82 3.11 4.25 1.58 2.97

Ro (%)

2.46 2.47 2.56 2.58 2.55 2.76 2.35 2.64 2.14 2.54 2.78 2.47 2.84

S1 (mg/g)

0.02 0.05 0.06 0.05 0.03 0.04 0.03 0.05 0.06 0.04 0.02 0.02 0.04

S2 (mg/g)

0.04 0.09 0.10 0.08 0.03 0.03 0.04 0.04 0.08 0.04 0.03 0.02 0.04

HI (mg/g)

1.57 2.97 2.47 3.40 1.26 1.60 1.73 2.05 2.84 1.29 0.70 1.26 1.34

Tmax (°C)

507 423 476 523 464 531 501 412 395 489 457 501 500

mature stage. The values of HI and S2 are quite low, indicating that a great deal of shale gas has been generated and the hydrocarbon generation potential of the Wufeng shale is quite poor.

Maceral composition (%) Sapropelinite

Inertinite

87 85 78 79 84 68 85 77 81 71 80 85 74

11 10 16 22 14 15 12 15 15 16 20 23 14

TI

76 75 62 57 70 53 73 62 66 55 60 62 60

Mineral composition (%) Qz.

Clays

Fel.

It.

I/S

55.6 48.6 65.3 49.2 54.7 44.6 51.3 42.3 67.2 62.3 69.8 50.5 61.6

22.3 20.3 16.1 21.4 15.3 26.5 20.9 19.8 15.6 17.5 10.6 17.6 18.3

10.2 12.3 7.8 9.8 15.2 13.5 17.8 10.6 15.1 6.8 14.7 15.5 12.2

14.9 20.0 14.9 10.3 8.7 24.2 18.0 18.2 14.8 16.0 10.1 13.0 15.0

7.4 0.3 1.2 11.1 6.6 2.3 2.9 1.6 0.8 1.5 0.5 4.6 3.3

very commonly observed in the Wufeng shales and are unlikely to contribute significantly to the overall porosity and permeability of the shales. The intra-pores within Type I quartz can be interpreted as the sites of fluid inclusions that dropped in the process of ion milling, which is in accordance with the observation of Li et al. (2018), and may provide robust evidence for Type I quartz being terrigenous origin. The CL-response of Type I quartz showed two intensity (count) peaks in the wavelength range of 390–430 nm and close to 700 nm (Fig. 6e). Such peaks are frequently observed for Type I quartz in the Wufeng shales and may be further evidence of a terrigenous origin. Type II quartz mainly presented as fine-grained microcrystalline quartz with a grain size that was generally less than 5 μm and commonly 1–3 μm (Figs. 7–9). The grains were subhedral to euhedral and occurred in both intragranular pores and intergranular matrix pores, likely acting as a cement. SEM-CL analysis showed that Type II quartz had dull or no luminescence (Fig. 9a–d), which may indicate an authigenic origin and is similar to Longmaxi shale in the Sichuan Basin and Eagle Ford shale (Zhao et al., 2017a; Milliken et al., 2016). The CLresponse of the Type II quartz commonly showed one broad intensity (count) peak in the wavelength range 580–650 nm (Fig. 9e), which has been indicated by Zhao et al. (2017a) and Thyberg et al. (2010) as being consistent with biogenic authigenic quartz. Most of the Type II quartz grains seem to inter-connect and are generally intergrown with

4.2. Quartz types and occurrence Three quartz types can be identified in the samples based on information derived from the SEM/SEM-CL and CL spectra regarding the morphology, distribution characteristics and CL intensity. Type I quartz had a relatively large grain size (commonly > 5 μm) ranging from 5 to 25 μm mostly (Figs. 4 and 5). In addition, most of Type I quartz greater than 35 μm were commonly observed in SY-2 and SY-3 (Fig. 4). Type I quartz primarily occurred as subangular to angular monocrystals (Fig. 4a–c, 5), and some grains showed corrosion (Fig. 4a and b). SEMCL analysis showed that Type I quartz exhibits bright luminescence of a relatively uniform intensity within each grain (Fig. 6a–d); this may indicate a terrigenous origin (Schieber et al., 2000; Zhao et al., 2017a; Xi et al., 2019). Type I quartz was generally floating in the shale matrix in close contact with other minerals (some grains were surrounded by clay minerals that showed significant deformation) (Fig. 5a). Thus, few inter-particle pores were observed between Type I quartz grains, though some intra-pores within Type I quartz grains were observed (Fig. 5). Such intra-pores were small and mostly isolated; these were not

Fig. 3. Representative X-ray patterns from two shale samples. 982

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Fig. 4. Characteristics of Type I quartz: a large grain size and irregular shape under SEM, natural cutting surface: (a) two large quartz grains, sample SY-2, (b) one large quartz grain with conchoidal fracture, sample SY-2 (c) one large quartz grain with a smooth surface, sample SY-3, and (d) spectrum from the red dot in (a) indicating an EDS point. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5. Characteristics of Type I quartz under SEM, polished surface: (a) quartz floating in the shale matrix, sample SY-8, (b)–(d) some intrapores can be observed within the Type I quartz, samples SY-6 and SY-9. 983

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Fig. 6. SEM images, SEM-CL images, and a CL spectrum of Type I quartz: (a) SEM image of several Type I quartz grains floating in the shale matrix, sample SY-8, (b) SEM-CL image of (a) showing the strong luminescence of Type I quartz. One quartz grain (at the top left) includes a region lacking bright luminescence that consists of Type III quartz; (c) SEM image of several Type I quartz grains floating in the shale matrix, sample SY-8, (d) SEM-CL image of (c) showing the strong luminescence of Type I quartz; (e) CL spectrum of Type I quartz.

Fig. 7. Characteristics of Type II quartz: a small grain size and subhedral to euhedral shape under SEM, natural cutting surface: (a)–(b) The micro-crystalline quartz is generally intergrown with OM, sample SY- 10, (c)–(d) spectra for the EDS points indicated by red dots in (a) and (b). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 8. Characteristics of Type II quartz under SEM, polished surface: (a) and (b) Small grain size Type II quartz is generally intergrown with OM, SY-10; (c) and (d) Type II quartz has a relatively large grain size in sample SY-11 compared to in SY-10; Q = quartz; OM = organic matter.

OM (Figs. 7–9). Unlike in Type I quartz, inter-particle pores between Type II quartz can be observed, but no intra-pores are seen (Fig. 7). The inter-particle pores had irregular pore shapes and widely pore size distributions (PSD), and most of the inter-particle pores have good connectivity. Type III quartz presented as quartz overgrowths and could only be identified by CL. Type III quartz mainly occurred along the outer part of Type I quartz, and SEM-CL analysis showed it to be essentially nonluminescent, indicating an authigenic origin (Figs. 6b and 10a–d). It was not found on every Type I quartz grain. Generally, if several Type I quartz grains were observed in one SEM-CL image, only one or two grains had associated Type III quartz. In addition, Type III quartz always appeared close to clay minerals (Milliken, 1994; Niu et al., 2018). There are two frequently observed patterns of CL response for Type III quartz in the Wufeng shales, both sharing a common feature (Fig. 10e and f). The CL spectra of Type III quartz were ‘bell-shaped’ and featured a lot of ‘noise,’ likely result from an additional signal from the surrounding shale matrix (probably clays) (Thyberg et al., 2010). To summarize, three quartz types were identified, each with a different morphology and grain size and potentially different origins and silica sources, and these may have different impacts on shale gas reservoirs. Terrigenous versus authigenic quartz can be discriminated by crystal shape and the spatial distribution and intensity of the CL response, but additional analysis methods are needed to discriminate the origins of authigenic quartz further, as discussed in section 5.1.

named the multilayer adsorption stage. The third stage is also multilayer adsorption, but when the relative pressures close to 1.0, the third stage featured a sharp increase in uptake (Li et al., 2018b; Xi et al., 2018b). The features of the N2 adsorption–desorption isotherms suggest that micropore, mesopore, and macropores are all developed in the Wufeng samples. The shales featured a large BET SSA (Brunauer–Emmett–Teller specific surface area) but a relatively small BJH PV (Barrett–Joyner–Halenda pore volume), and the two values exhibited a weak positive relationship (Fig. 12). These findings may indicate that micropore and fine mesopore may be dominated pore types for the Wufeng shale because micropore and fine mesopore contribute more SSA than PV. These relationships are in accordance with the PSD of the samples (Fig. 11b). Pores of 2–6 nm are the main contributors towards the PV, followed by pores larger than 30 nm in size. The porosity of the 13 samples ranged from 2.13% to 4.69% with an average of 2.92%, and their permeability ranged from 0.00122 mD to 0.0446 mD. These values are larger than the limits of porosity (around 1%) and permeability (around 0.001mD) for a shale gas reservoir (Nie et al., 2011). Porosity was positively correlated with permeability (Fig. 13), indicating that the pores making up the porosity may have good connectivity.

4.3. Pore structure

In general, the quartz can be classified in terms of origin into two types, namely terrigenous quartz and authigenic quartz, both of which have many potential silica (Si) sources (Table 2). As discussed in section 4.2, Type II quartz is the most commonly observed type in the Wufeng shales and likely has an authigenic origin. A negative linear relationship between SiO2 and zirconium (Zr) or Ti can be observed in the samples (Fig. 14), also indicating that most of the quartz grains are authigenic rather than detrital (Ross and Bustin, 2009; Gambacorta et al., 2016). Moreover, many studies have suggested that quartz in marine shales

5. Discussion 5.1. Quartz origins and silica source

The N2 physisorption curves displayed an obvious hysteresis loop (Fig. 11a) that can be characterized as a mixture of types H2 and H3 in the IUPAC classification. The shape of the N2 physisorption curves showed that the pores are dominantly inkbottle-shaped and slit-shaped (Xi et al., 2017a). The N2 adsorption process in shales has three stages. The first stage is at relative pressures less than 0.01 and is termed the micropore filling stage. The second stage is between 0.4 and 0.8 and is 985

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Fig. 9. SEM images, SEM-CL images, and a CL spectrum of Type II quartz: (a) SEM image of abundant Type II quartz in sample SY-10, (b) SEM-CL image of (a) showing the dull or non-luminescence of Type II quartz; (c) SEM image of Type II quartz in sample SY-11, (d) SEM-CL image of (c) showing the dull luminescence of Type II quartz; (e) CL spectrum of Type II quartz.

from south China is primarily authigenic (Liu et al., 2018; Zhao et al., 2017a; Yang et al., 2018). Thus, this study discusses the origin of and potential silica sources for authigenic quartz in further detail. As shown in Table 2, the silica in the authigenic quartz in the shale has several possible sources: alteration of biogenic silica, defined as biogenic quartz, clay mineral transformation, defined as diagenetic quartz, and others. The Al–Fe–Mn diagram (Adachi et al., 1986) is generally used to characterize the formation environment of chert. All the samples fall into the non-hydrothermal field (Fig. 15), indicating that the contribution of a hydrothermal Si source may be minimal for the Wufeng marine shales. In addition, there are not many obvious hydrothermal minerals observed in the Wufeng shale, which may indicate that there is no hydrothermal activity in the study area (Wan et al., 2017). There are no ash beds in well SY5, and there are few known volcanic ash beds in the study area (Wang et al., 2018; Wan et al., 2018; Xi et al., 2018b). Further, high Rb/K2O ratios indicate ancient and highly weathered sources, whereas low Rb/K2O ratios indicate a major volcaniclastic component (McLennan et al., 1990; Plank and Langmuir, 1998). The Rb/K2O ratios of the target samples are close to those of Post-Archean Average Shale (Fig. 16). Thus, the upper continental crust may have provided the sediments making up the Wufeng shale (Zhao et al., 2017a), and the shales may contain a negligible amount of silica

sourced from alteration of volcanic ash. Although the Wufeng shale contained abundant rigid minerals, grain-to-grain contacts were rarely observed, and thus the pressure dissolution of silica source may be minimal for the Wufeng marine shales. During initial deposition, the sea surface was rich in paleo-organisms that could provide abundant biogenic silica such as algae and siliceous organisms (radiolarians, spongy bone needles, and diatoms), especially in the Wufeng and Lower Member Longmaxi Formations (Zhang et al., 2018). There is an obvious positive correlation between TOC and quartz content for the Wufeng shale (Fig. 17a), and the excess Si ranged from 8.73% to 65.69% with an average of 37.94% (Table 3; Wedepohl, 1971), which also positively correlated with the TOC (Fig. 17b). In addition, the ration of Si/(Si + Fe + Al + Ca) averages 0.81 in the Wufeng shale (Table 3), which is similar to the average value of 0.82 for fifteen biogenic chert samples and the average ratio of 0.80 in Wufeng and Longmaxi Formations of JY2 well in the Jiaoshiba area (Ran et al., 2015; Zhao et al., 2017a), further indicating that most of the quartz is biogenic in origin (Rangin et al., 1981; Ruiz-Ortiz et al., 1989). Moreover, the Type II quartz (the most commonly observed type) has a small grain size (commonly 1–5 μm), which is similar to the sizes of opal lepispheres and their quartz recrystallization products (Jones and Renaut, 2007), and the morphology, distribution characteristics and CL intensity of the Type II quartz is similar to quartz that 986

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Fig. 10. SEM images, SEM-CL images, and CL spectra of Type III quartz: (a) SEM image of a large quartz grain in sample SY-1, (b) SEM-CL image of (a) showing Type III quartz with dull or non-luminescence around one large quartz grain; (c) SEM image of a large quartz grain in sample SY-2, (d) SEM-CL image of (c) showing Type III quartz with dull luminescence; (e)–(f) two common CL spectra of Type III quartz.

has been interpreted as of biogenic origin in south China Longmaxi shales (Zhao et al., 2017a) and other mudstones (Jones and Renaut, 2007; Harris et al., 2011). This morphological evidence may further support the identification of biogenic silica as the principal source of authigenic quartz in these shales. As many have previously mentioned, silica can be released during alteration of clay minerals (Srodon, 1999; Peltonen et al., 2009).

Metwally and Chesnokov (2012) used laboratory experiments to show that authigenic quartz can be formed through clay mineral transformations and determine theoretically that an original bulk rock smectite content of about 25% would produce about 5% additional quartz (Boles and Franks, 1979). As illite is the major type of clay mineral in the shales studied here, sericitization of illite (the I-S reaction) may be the dominant diagenetic process altering clay minerals in the Wufeng

Fig. 11. (a) N2 physisorption curves and (b) PSD of representative shale samples. 987

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XRD method. If KI positively correlated with quartz content, the silica releasing from alteration of clay minerals may be the principal source of authigenic quartz as indicated by Yang et al. (2018). However, the relationship is weak for the Wufeng samples (Fig. 19). (3) The morphology and grain size of Type II quartz (the most commonly observed type) is not same as the “sheet-like” quartz that generally resulted from the alteration of clay minerals (Thyberg and Jarhren, 2011). Type III quartz may be mostly formed through the I-S reaction because clay minerals can be observed near the quartz overgrowths. In addition, Niu et al. (2018) found on the basis of electron probe micro-analysis (EPMA) that quartz overgrowths in marine shales have a relatively high content of Al, which is derived from clay mineral reactions. However, the Type III quartz is more rarely observed than the other two types: if an SEM image contains several quartz grains, only one or two will have a quartz overgrowth. (4) The Wufeng shale contained relatively low clay minerals, and an original bulk rock smectite content of about 25% would only produce about 5% of additional quartz theoretically, as mentioned above.

Fig. 12. Relationship between BET SSA and BJH PV based on the N2 adsorption results.

In summary, the silica was mainly sourced from two processes: dissolution of siliceous organisms and the I-S reaction, with the biogenic origin being the most important. According to the geochemical analysis, the percentage of different origin of quartz can be roughly calculated through the two equations (Wedepohl, 1971; Van, 2008; Liu et al., 2017).

Si excess = Sisample

[(Si/Al) background

Al sample]

(1)

1.308[(Al3.15Mg0.85)(Si8.00)O20(OH)4(Na0.85)2H2O[smectite] + (0.06Fe2O3 + 0.56K2O + 0.02CaO)] = [(Al4.12Fe0.1Mg0.56)(Si7.17) O20(OH)4(K1.47Na0.01Ca0.03)][illite] + 3.29SiO2 + 0.56Na2O + 0.55MgO + 3.23H2O (2) The detailed process by which the percentage of quartz with different origins can be derived and some important assumptions are discussed by Niu et al. (2018) and Yang et al. (2018). Their method was adopted for the current study. The results show that the Wufeng shale contains an average of 48% biogenic silica, 6% digenetic silica, and 46% detrital silica (Table 4).

Fig. 13. Relationship between the porosity and permeability of the Wufeng shale samples.

shales. The alteration of smectite to illite requires abundant potassium and certain temperature and pressure conditions. The Wufeng shale has high thermal maturity and thus the sericitization of illite has been occurred, and the corrosion of feldspars, which can provide abundant potassium, can be observed in the shales (Fig. 18). Therefore, the silica released by clay mineral alteration (the I-S reaction) may be another potential source of authigenic quartz. However, several lines of direct and indirect evidence indicate that the principal source of authigenic quartz in Wufeng shales is not from the sericitization of illite:

5.2. Timing of quartz formation In general, diagenesis can be divided into initial deposition, early diagenesis, middle diagenesis, and late diagenesis (Loucks et al., 2012). As discussed in section 5.1., most observed authigenic quartz are biogenic, and some is sourced from the S-I reaction. Several lines of direct and indirect evidence indicate that the biogenic quartz was mainly formed during shallow burial in early diagenesis and that the diagenetic quartz was mainly formed during middle diagenesis. The timing of quartz-formation by different processes will have had significant effects on reservoir quality (these effects will be discussed in detail in sections 5.3 and 5.4).

(1) Excess Si has positive correlation with TOC but negative relationship with Al2O3 (Fig. 17), which may indicate that the principal source of authigenic quartz is from siliceous organisms rather than sericitization of illite. (2) The Kubler index (KI), which represents the crystallinity of illite (Kübler, 1969; Kübler and Jaboyedoff, 2000), can obtain from the Table 2 Quartz origins and potential silica sources in mudstone. Origin Terrigenous quartz Authigenic quartz

Potential silica source 1. Fine-grained low-grade metamorphic rocks 2. Fracturing of coarser grains during soil formation and weathering 3. Abrasion during transport by water or wind 1. Alteration of biogenic silica (Zhao et al., 2017a; Yang et al., 2018); 2. Clay mineral alteration (sericitization of illite, illitization of muscovite, illitization and chloritization of kaolinite) (Peltonen et al., 2009; Thyberg et al., 2010; Thyberg and Jahren, 2011) 3. Pressure dissolution of silica minerals (quartz or feldspar) (Füchtbauer, 1978) 4. Hydrothermal silica input (Li et al., 2018a) 5. Alteration of volcanic ash

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Fig. 14. Relationships between (a) SiO2 and TiO2 and (b) SiO2 and Zr.

1982). It is generally accepted that biogenic opal is very unstable and dissolves easily as burial depth increases in early diagenesis (Williams et al., 1985). Spinelli et al. (2007) pointed out that opal-A and opal-CT are already abundant when burial depth is less than 200–300 mbsf (meter below the seafloor), and that almost all the opal has already been converted to quartz before reaching 500 mbsf. Using the isotopic method, Matheney and Knauth (1993) revealed that biogenic quartz in the Miocene Monterey Formation has been generated at 17–21 °C. In addition, the characteristic of biogenic quartz of having dull or no luminescence under SEM-CL is in accordance with low/non-luminescent quartz commonly interpreted as being of low-temperature authigenic/diagenetic origin in sandstone (Zinkernagel, 1978). As mentioned in section 4.1, our best explanation of the small peak in sample SY-5 (Fig. 3) is that it is a remnant of the properties of opal-A or opal-CT. This would be possible through the incorporation or isolation of metastable opalA/CT in the quartz cement during the local re-crystallization process. Thus, it is indicated that the biogenic quartz formed in very early diagenesis, allowing opal-A/CT to survive in Wufeng shale that has been buried to a depth beyond 6000 m (Wan et al., 2017). Furthermore, theoretical research carried out on the basis of geochemical analysis indicates that siliceous organisms can be dissolved and re-precipitated to form biogenic quartz in early diagenesis (Schieber, 1996). Thus the biogenic quartz is considered to have formed at low temperature at shallow burial depths. (2) Most of the biogenic quartz was pore-filling, indicating precipitation at depths consistent with porosity at least as high as the volume of the pore-filling biogenic quartz. In addition, the biogenic quartz size is commonly 1–3 μm, indicating that the shale should contain a great deal of pores with large diameters at the formation time of the biogenic quartz. Mechanical compaction starts immediately after deposition and mainly occurs at temperatures below 70 °C and thus at depths shallower than 1–2 km (Mondol et al., 2007). In general, porosity typically declines from an initial value near 30–80%, and the size of intergranular matrix pores reduces significantly over the first 1000 m of burial (Milliken and Day-Stirrat, 2013). Therefore, biogenic quartz must have formed before significant compaction and at a relatively early stage. Work by Niu et al. (2018) showing that shale maintained 70–80% porosity when biogenic quartz formed in sponge spicules, by Spinelli et al. (2007) suggesting that biogenic quartz formation occurred in shale with at least 59% porosity, and by Schieber (1996) arguing that algal cysts can keep their shape after severe compaction are also supportive of an early timing for silicification. (3) Biogenic quartz has often been observed in shallowly buried (< 300m) and immature shale samples (Spinelli et al., 2007; Millilken and Day-Stirrat, 2013; Millilken et al., 2016), further confirming that biogenic quartz can form during shallow burial in early diagenesis. However, no diagenetic quartz sourced from the S-

Fig. 15. The shale samples plot in the non-hydrothermal area in the Al-Fe-Mn diagram.

Fig. 16. Relationship between Rb and K2O for Wufeng shale samples.

(1) Biogenic quartz originates from dissolution and re-precipitation of biogenic silica that was mainly present in the organisms as biogenic opal (SiO2·nH2O). The formation process for biogenic quartz involves the alterationof siliceous ooze (opal-A) derived from siliceous organisms to opal-CT and then its recrystallization as quartz (Rice et al., 1995). Alteration to opal-A, opal-CT and quartz are functions of thermal history and reaction kinetics (Isaacs, 1981, 989

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Fig. 17. Relationships between (a) TOC content and quartz content, (b) TOC content and excess Si, and (c) Al2O3 and excess Si.

I reaction has been observed in these shallowly buried and immature shale samples, indicating that biogenic quartz generated at much early in the burial process than that of diagenetic quartz. In general, the S-I reaction occurs during progressive burial at temperatures between 60 °C and 100 °C, mainly at around 80 °C during middle diagenesis (Peltonen et al., 2009). Therefore, diagenetic quartz precipitation was impossible during early diagenesis because temperatures were too low for significant illitization of smectite (Metwally and Chesnokov, 2012). Thyberg et al. (2010) reported observations of diagenetic quartz at temperatures exceeding 80–85 °C in two wells but no evidence showing diagenetic quartz formation in shale with a high content of smectite at shallow depths (temperatures below about 60–70 °C). In addition, the alteration of smectite to illite requires abundant potassium, which is most conveniently sourced from the dissolution of feldspars (Thyberg et al., 2010; Thyberg and Jahren, 2011). The dissolution of feldspars in shales is generally related to the decarboxylation of kerogen to

produce an increase in carboxylic and phenolic acids (Xi et al., 2017b, 2018d), and this generally occurs in the oil window (80–120 °C). Thus, the temperature range of the S-I reaction corresponds to the oil generation peak, further indicating that it would have been restricted to middle diagenesis. 5.3. Effect of quartz on pore structure Generally, the Wufeng samples had an initial porosity of more than 60%. Primary interparticle pores may have been the most common pore type when the shale was initially deposited. Porosity typically declines from an initial value near 30–80%, and contacts between minerals transform from spot-line to concave-convex as the burial depth increases during early diagenesis due to compaction (Mondol et al., 2007; Milliken and Day-Stirrat, 2013). Furthermore, most mudstones compact more than does experimentally compacted mud at equivalent effective stresses (Velde, 1996; Nygard et al., 2006; Peltonen et al., 2009), which

Table 3 The results of XRF analysis and elemental ratios of the target shale. Sample ID

Na2O%

MgO%

Al2O3%

SiO2%

P2O5%

K2O%

CaO%

TiO2%

MnO%

Fe2O3%

Excess Si

Si/(Si+ Fe +Al + Ca)

SY-1 SY-2 SY-3 SY-4 SY-5 SY-6 SY-7 SY-8 SY-9 SY-10 SY-11 SY-12 SY-13

0.72 1.10 0.54 1.07 0.75 1.06 0.75 0.98 0.47 0.55 0.35 0.72 0.82

1.58 2.91 1.13 2.27 1.56 2.91 1.88 3.25 1.26 1.39 0.85 2.49 1.89

10.17 8.90 7.11 13.06 9.96 14.76 10.28 15.46 6.63 8.20 5.52 11.47 9.53

70.15 60.65 79.61 63.39 68.69 59.98 67.92 56.82 78.52 75.05 82.86 66.38 74.94

0.09 0.13 0.19 0.11 0.23 0.12 1.31 0.11 0.09 0.44 0.12 0.07 0.07

2.69 4.42 1.92 3.48 2.55 4.52 2.65 4.19 1.81 2.27 1.71 3.21 2.40

2.07 0.25 0.64 1.86 1.58 0.29 3.03 2.09 0.96 1.06 0.61 2.04 1.11

0.40 0.68 0.32 0.56 0.45 0.68 0.48 0.63 0.30 0.37 0.22 0.55 0.42

0.04 0.06 0.03 0.06 0.04 0.06 0.05 0.14 0.03 0.04 0.03 0.08 0.05

3.60 6.95 2.99 5.59 5.53 7.18 3.66 6.76 3.27 4.58 3.00 4.80 4.15

38.52 32.95 57.5 22.76 33.71 14.07 35.94 8.73 57.87 49.53 65.69 30.69 45.31

0.82 0.79 0.88 0.76 0.8 0.73 0.8 0.7 0.88 0.85 0.9 0.79 0.84

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Fig. 18. SEM images showing (a) the corrosion of K-feldspar and (b) the dissolution K-feldspar.

properties due to containing more biogenic quartz than is usual or from having an originally better pore system. Based on our results, the biogenic quartz content was positively correlated with the porosity (Fig. 20e and f). There are two possible reasons for this. (1) More biogenic quartz formed, so more primary pore space was preserved. (2) The shale contained more biogenic quartz, which may indicate that the shale contained more TOC. In general, a reducing sedimentary environment and high paleo-production favor the formation of biogenic quartz (Ross and Bustin, 2009; Du et al., 2012), and these conditions may also favor OM accumulation and preservation (Li et al., 2018b). The second point may be more important, because the biogenic quartz more likely acted as a pore protector and the OM more likely act as a pore provider. However, further discussion of the relationship between quartz origin and enrichment in OM is out the scope of this study. Diagenetic quartz sourced from clay mineral alterationusually acts as a cement that reduces porosity and causes a significant change in rock stiffness during middle diagenesis and even late diagenesis (Peltonen et al., 2009; Thyberg and Jahren, 2011), but the amount of clays in the shale will determine the potential for diagenetic quartz cement formation and thus the degree of chemical compaction. The clay content of the Wufeng shales in this study is relatively low, and the percentage of diagenetic quartz is much lower than of other types, which may mean that diagenetic quartz had little effect on the pore structure. For marine shales with a relatively high clay mineral content, most of authigenic quartz may source from alteration of clay minerals. In such cases, the diagenetic quartz may lead the shale gas reservoir to have lower porosity and poor pore connectivity. In addition, the clay mineral type also has an important effect on the potential for diagenetic quartz formation. If the shale contains more stable silica-rich clays, these will be hard to transform to release the silica during diagenesis. For example, marine-continental transitional shale contains abundant clays (generally above 40%), and kaolinite is one of the major clay types, but most of the quartz is of terrigenous origin (Xi et al., 2017a, b). Many studies suggested that BET SSA and BJH PV that obtained by N2 adsorption are strongly influenced by clay minerals (Wang et al., 2016). However, no obvious relationships are observed between clay minerals and pore structure parameters for the Wufeng shales (Fig. 20g and h), indicating that the effect of clays content may be low. In addition, Xi et al. (2017b) pointed that the shale with abundant TOC and quartz make dominated contributions to the pore system (Zhao et al., 2016), and the Wufeng shales contained a lot of quartz and comparable TOC content but relative little clays.

Fig. 19. Relationship between KI value and quartz content.

may indicate that chemical compaction may combine with mechanical compaction to reduce porosity in this period. As discussed in section 5.2., a large amount of biogenic quartz formed in very early diagenesis, which would have filled in the primary pore space as a cement and decreased the original pore volume. But on the other hand, the authigenic biogenic quartz will have acted as a rigid framework that prevented the pore space from compacting further (Fishman et al., 2013). Therefore, there are still some pores related to biogenic quartz that can be observed (Fig. 5a and b), but most of the pores preserved by the biogenic quartz have been filled by OM in the oil window actually (Fig. 5c and d); this may be the reason why the OM is generally intergrown with quartz. This also means that the early-formed quartz controlled the distribution of migrated OM and further controlled the development of OM pores. The quartz has a positive relationship with porosity and BJH PV (Fig. 20a and b), but PV and porosity have a much stronger relationship with TOC that does quartz (Fig. 20c and d), which may indicate that biogenic quartz are not the main pore provider but acted as a pore protector. Although some inter-particle pores exist between the quartz grains, the positive correlation between TOC and quartz may be one of the primary reasons that quartz positively correlated with the porosity and pore volume. The biogenic quartz prevented the primary pores from compacting further in the burial process, and thus OM can fill the pore space preserved by biogenic quartz in the oil window. With the increase in depth, the OM that filled in the pore space preserved by biogenic quartz in the oil window is the principal matrix for OM pore development, and the quartz framework have also provided pressure shadows for the formation of OM pores. Thus, if quartz grains were not formed in shallow burial during early diagenesis, most of the primary pores in a shale would be compacted and OM pores forming during the period of hydrocarbon generation would also struggle to survive without the protection of a rigid mineral (quartz) framework (Fishman et al., 2012; Li et al., 2018d). Further discussion is required to determine whether this shale derived its advantageous

5.4. Impact of quartz on rock mechanical properties Shale gas reservoirs have low porosity and low permeability, making the exploitation of shale gas more difficult (Cadwallader et al., 2015; Wu et al., 2018). In addition, shale gas cannot be produced naturally but economical, and effective productivity can be achieved 991

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Fig. 20. Relationships of TOC content and quartz content with BJH PV and porosity.

through fracturing. The brittle minerals content is an important parameter to evaluate fracability of a shale (Haris et al., 2011). Quartz is the dominant brittle minerals of the Wufeng shales, and quartz content closely correlated with the Poisson's ratio and Young's modulus (Fig. 21). Generally, shales with a high Young's modulus and low

Poisson's ratio are more brittle and have good fracability, whereas those with a low Young's modulus and high Poisson's ratio are more ductile, and fracturing them is inefficient (Rickman et al., 2008). Therefore, the quartz content of the Wufeng shales is an indication not only that it will be possible to initiate and emplace hydraulic fractures but also that the 992

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Fig. 21. Relationships among quartz content and rock mechanical parameters.

Fig. 22. Correlations between BET SSA and Langmuir volume and TOC and quartz content.

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Table 4 Calculated percentage of quartz in different origins for the Wufeng shale samples basing on the geochemical data. Sample ID

SY-1 SY-2 SY-3 SY-4 SY-5 SY-6 SY-7 SY-8 SY-9 SY-10 SY-11 SY-12 SY-13

Terrestrial SiO2

31.62 27.68 22.10 40.62 30.98 45.91 31.98 48.08 20.63 25.51 17.16 35.68 29.62

Biogenic SiO2

34.78 27.93 53.76 20.18 35.53 8.00 31.42 4.16 53.36 45.52 61.42 27.43 41.54

Diagenetic SiO2

3.74 5.02 3.74 2.59 2.18 6.08 4.52 4.57 4.52 4.02 4.27 3.26 3.77

fractured shale will then maintain its long-term conductivity. However, the many studies that have directly evaluated shale fracability based on the brittle mineral content or quartz content have not specifically considered the form of the minerals (Jarvie et al., 2007; Fang and Amro, 2014; Xi et al., 2017a, b). The terrigenous quartz in the shales studied here (Type I) was floating in the matrix and did not act as a cement, unlike the authigenic quartz, potentially indicating that the terrigenous quartz contributes little to giving the rock brittle behavior. In contrast, the authigenic quartz acts as a grain-binding network in the shale matrix, which may be expected to generate brittle behavior. The authigenic quartz content has a stronger positive relationships with the Young's modulus and Poisson's ratio than do the total quartz content and terrigenous quartz content (Fig. 21c and d), indicating that quartz with different origins and different distribution characteristics and shapes has different effects on shale fracability. The bulk content of quartz alone is not a guarantee of brittle behavior. We have thus identified a new factor to take into account when evaluating the brittleness or fracability of a shale gas reservoir, which may have some theoretical value for evaluating the rock mechanical properties of shales.

Relative percentage (%) Terrestrial SiO2

Biogenic SiO2

Diagenetic SiO2

45.08 45.65 27.76 64.08 45.10 76.54 47.08 84.63 26.28 33.99 20.72 53.75 39.53

49.58 46.06 67.54 31.84 51.72 13.33 46.26 7.32 67.96 60.65 74.13 41.33 55.44

5.34 8.28 4.69 4.08 3.18 10.13 6.66 8.05 5.76 5.35 5.15 4.92 5.03

Type I quartz generally floats in the shale matrix, is relatively coarse-grained and exhibits bright luminescence, Type II quartz generally has a small grain size and inter-connect and intergrown with OM and shows dull or non-luminescence, and Type III quartz mainly occurs as a dark, irregular rim on Type I quartz grains. (2) The origin of the quartz has been determined through analysis of the petrologic characteristics and geochemistry of the Wufeng shale samples. Type I quartz may be of terrigenous origin, whereas Type II and III quartz may be authigenic. Further, Type II quartz may be of biogenic origin, with the silica likely sourced from the dissolution of siliceous organisms, mostly during early diagenesis. Type III quartz may be sourced from the alteration of clay minerals (the illite-sericite reaction), probably mostly during middle diagenesis. Authigenic quartz is dominantly biogenic in the Wufeng shales. (3) Different types of quartz with different origins and formation times had different impacts on the shale gas reservoir. The authigenic quartz generally acted as a cement, and shale with a high content of authigenic quartz had a high Young's modulus and low Poisson's ratio, contributing considerably to the brittleness of the shale. The rigid framework of biogenic authigenic quartz prevented pore compaction, controlling the distribution of OM and the development of OM pores. Shale that was high in biogenic quartz had high adsorption capacity and porosity and therefore high storage capacity.

5.5. Impact of quartz on adsorption capacity The Langmuir volume of the Wufeng shale ranged from 0.87 m3/t to 2.15 m3/t, and TOC content positively correlated with the BET SSA as well as Langmuir volumes (Fig. 22a and b), which was in accordance with previous studies of marine shales (Xi et al., 2018a, b). We note that the biogenic quartz also has positive relationships with BET SSA as well as Langmuir volumes (Fig. 22c and d). The biogenic quartz has small grain size and large SSA (Fig. 7a and b, 8a, c) and thus can provide more adsorption sites. In addition, the inter-particle pores related to biogenic quartz had smaller pore diameter and may lead to greater adsorption. Note that the Langmuir volume had a much stronger relationship with TOC that does biogenic quartz, and the positive correlation between TOC and biogenic quartz may be one of the primary reasons that biogenic quartz positively correlated with the Langmuir volume, which may indicate that TOC is the primary controlling factor. The biogenic quartz in marine shales can work together with OM to strengthen the adsorption capacity, and shale with abundant OM and biogenic quartz may have higher adsorption capacity, which was favorable for gas accumulation.

Acknowledgments The authors would like to thank National Science and Technology Major Project of China (grant no. 2017ZX05035001). We also greatly thank the anonymous reviewers and editors for their critical comments and valuable suggestions, which were very helpful to improve the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.04.051. References Adachi, M., Yamamoto, K., Sugisaki, R., 1986. Hydrothermal chert and associated siliceous rocks from the northern Pacific: their geological significance and indication of ocean ridge activity. Sediment. Geol. 47, 125–148. Blatt, H., Schultz, D.J., 1976. Size distribution of quartz in mudrocks. Sedimentology 23, 857–866. Boles, J.R., Franks, S.G., 1979. Clay diagenesis in Wilcox sandstones of Southwest Texas: implications of smectite diagenesis on sandstone cementation. J. Sediment. Petrol. 49 (1), 55–70. Buckman, J., Mahoney, C., März, C., Wagner, T., Blanco, V., 2017. Identifying biogenic silica: mudrock micro-fabric explored through charge contrast imaging. Am. Mineral.

6. Conclusions (1) Quartz is the most important and common mineral in the Wufeng shales of the northwest Hunan Province, south China. Three types of quartz can be identified based on the SEM/SEM-CL analysis. 994

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